1. Field of the Invention
This invention relates generally to implantable medical devices, and, more particularly, to methods, apparatus, and systems for providing a safe-mode operation of the implantable medical device using a dynamic impedance adjustment process.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated in its entirety herein by reference in this specification. Electrical stimulation of the vagus nerve (hereinafter referred to as vagus nerve stimulation therapy or VNS) may be provided by implanting an electrical device underneath the skin of a patient and performing a detection and electrical stimulation process. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy, and may periodically apply a series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
Generally, therapeutic electrical stimulation is delivered by the implantable device via a lead. The lead generally terminates onto an electrode, which may be affixed onto a tissue. A plurality of electrodes that are associated with an implantable medical device are generally operatively connected to the implantable device via individual leads. Therefore, a number of leads may project from the implantable device onto various portions of a patient's body. For example, a number of electrodes may be attached to various points of a nerve or other tissue inside a human body.
Occasionally, a patient having an implantable device may be subjected to an electrical field, a magnetic field, and/or an electromagnetic field. In the proximity of one of the aforementioned fields, coupled signal and/or noise may appear on various portions of the implantable device system, particularly on the leads. Depending on the strength of the field, a significant amount of coupled energy may appear on the leads. This coupled energy may cause adverse effects. For example, the coupled energy on the leads may affect operation of the device, or cause adverse thermal changes. The coupled signal or energy may also interfere with the delivery of the electrical/magnetic stimulation therapy, or with the proper detection of various signals from the electrodes. Other adverse effects, such as heating of various portions of the implantable system may occur. This heating may damage tissue that is proximate to the portion of the implantable system that experiences thermal the changes.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.